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VITED STATES DEPARTMENT OF AGRICULTURE 

BULLETIN No. 969 

Contribution from the Bureau of Animal Industry 
JOHN R. MOHLER, Clilef 




^•\J^''^>J'L. 




Washington, D. C. 



PROFESSIONAL PAPER 



October 6, 1921 



THE MEXICAN WHORLED MILKWEED (ASCLEPIAS 
MEXICANA AS A POISONOUS PLANT. 



.V. 



By C DwiGHT Marsh, Phybiologist m Charge of Investigations of Stock Poisoning by 
Plants, and A. B. Clawson, Physiologist, Pathological Division. 



CONTENTS. 



Page. 

Introduction 1 

Description of A sclepias meiicana 3 

Distribution and habits of A . meiicana 3 

Experimental work 4 

Typical case, Sheep 610 5 

Results of experimental work and conclusions . 7 

Symptoms 7 

Delay in development of sjrmptoms 7 

Duration of sickness 9 



Results of experimental work, etc.— Contd. 

Autopsy findings 9 

Pathological changes in tissues 10 

Toxic and lethal doses 13 

Comparative toxicity of species of whorled 

milkweed 15 

Animals affected 16 

Summary '. 16 

Literature cited 16 



INTRODUCTION. 

Many of the milkweeds are said to have poisonous properties. 
Definite experiments, however, proving their toxic or nontoxic 
characteristics have been made in regard to only a few species. 
Reports by stockmen and others of the poisoning of livestock by 
these plants have been made from many parts of the Western States, 
and yet almost an equal number of reports have been made asserting 
that the plants are harmless. Generally speaking, the reports of 
harmful results have not indicated clearly what species of milkweed 
was supposed to be involved. For the most part the plants were not 
examined by botanists who could determine the species correctly. 
The United States Department of Agriculture, in making investiga- 
tions of these cases, has frequently been very much in the dark as to 
the actual plant suspected and has been led to make experiments on 
plants which proved to be nontoxic so far as livestock are concerned. 

Certain definite and rather startling results, however, have been 
obtained in the investigation of the whorled milkweeds. As shown 
in Bulletin 800 of the United States Department of Agriculture, 

48677°— 21— Bull. 969 



"a 



?. 



2 BULLETIN !)6f>, U. S. DEPARTMENT OF AGRICULTURE. 

Asclepias galioides, the whorled milkweed of Colorado, Utah, Arizona, 
and New Mexico, is one of the most poisonous of the weeds affecting 
our livestock, and under sopae circumstances causes very heavy 
losses. Especial interest, therefore, attaches to the other closely 
allied plants. The whorled milkweeds form a very distinct group of 
the milkweed family, Asclepiadacese, being easily recognized by the 
narrow leaves which are frequently arranged in a whorled manner. 

The number of species of these plants depends upon the opinions 
of the botanists who study them, but for our purposes four may be 
recognized which have a fairly definite geographical range. The 
relationships of these species are briefly characterized by Eggleston in 
Bulletin 800, United States Department of Agriculture, pages 5 and 6: 
Asclepias galioides is found in Colorado, Utah, Arizona, and New 
Mexico, west of the Continental Divide, and so far as known does not 
extend west of Utah; Asclepias pumila grows in the plains east of 
the Rocky Mountains; Asclepias verticillata is found in the Mississippi 
Valley, and eastward; Asclepias mexicana is a western species, 
being especially abundant in California. 

In Bulletin 942, United States Department of Agriculture, Asclepias 
pumila and Asclepias verticillata var. geyeri were shown to have 
properties similar to those of Asclepias galioides, but differing in 
degree. It is evidently a matter of considerable importance, as weU 
as of interest, to find out definitely the character of Asclepias mexicana, 
which has been frequently reported in California as a sheep-poisoning 
plant, although published statements are very few. 

Chesnut (1898) says that it is reported from California that sheep 
and calves were poisoned by eating the growing plant, and cows by 
eating hay containing it. 

Pammel (1910) quotes the statement of Chesnut. 

Jepson (1911) states that it is said to poison cattle. 

Hall (1912) says: 

It is much dreaded by sheepmen on hot days, when they are obliged to drive their 
flocks over dry districts, where there is little other vegetation and no water. Under 
these conditions the sheep are tempted to eat the milk>veed, which contains an active 
poison and causes many deaths. This is sometimes prevented by cutting the plants 
down a few days before the sheep are driven through, the object being to dry the 
herbage and so render it less tempting. 

Hall and Yates (1915) speak of the three species, Asclepias mexicana, 
Asclepias eriocarpa, and Asclepias speciosa, as being equally poisonous, 
treat of their distribution, and give symptoms as reported to them. 
These symptoms do not correspond to those appearing in sheep 
poisoned by Asclepias mexicana. It may be noted, too, that the 
authors of this bulletin have not only failed to get any clear evidence 
of poisoning by Asclepias speciosa, but have also failed experimentally 
to poison animals by it. 






V ' MEX 



ICAN WHOELED MILKWEED AS A POISONOUS PLANT. 3 

Fleming (1920) gives the dosage by which experimental calves and 
sheep were poisoned. 

Fleming and co-authors (1920) give a somewhat extended account 
of the plant, with a report of feeding experiments on cattle and sheep. 

It seems probable from present investigations that many of the 
cases of poisoning attributed to A. speciosa, A. eriocarpa, and A. 
fremonti were really due to A. mexicana. No detailed experimental 
work, however, has been done on A. eriocarpa or A. fremonti. 

DESCRIPTION OF ASCLEPIAS MEXICANA.' 

The following is a technical description of the plant, Asclepias 
mexicana (see PI. I), known as the Mexican whorled milkweed, also 
as the narrow-leaf milkweed. The stems are erect, single or several, 
often branching at base, woody at base, 1 to 6 feet high; main root 
horizontal, branching, producing adventitious buds; stems glabrous 
below, slightly short, hairy, and branching above; the leaves in 
whorls, 2 to 6, sometimes in axillary bundles, linear to narrowly 
lanceolate, short petioled, 2 to 6 inches long, 2 to 6 lines wide, often 
folding together, smooth, acute at both ends; flowers usually in 
corymbose terminal umbels, many-flowered, peduncles much shorter 
than leaves, and 2 to 3 times as long as the pedicels, which are sur- 
rounded by numerous narrow bracts; corymb and flowers short, soft, 
hairy; flowers greenish- white, sometimes purplish; calyx and corolla 
turned back after opening; calyx five-cut; corolla deeply five-cleft; 
crown of 5 hoods, hoods erect, open, containing a linear horn 2 or 3 
times as long as the hood, and surrounding the column (united 
stamens) in which are the 2 ovaries, with slender styles, which unite 
to the single diskliko stigma, the anthers adhering to the stigma and 
opening on the outside, thus compelling the use of insects or some 
other outside agency to transport the waxy mass of pollen to the 
stigma disk; pods from 2 to 3 inches long, smooth, narrow, splitting 
on the sides, seeds flat, reddish-brown, with a tuft of long hairs at 
summit. It flowers from June to August. 

DISTRIBUTION AND HABITS OF ASCLEPIAS MEXICANA. 

The plant ranges from Mexico northward through California, west- 
ern Nevada, and southern Washington to eastern Idaho, as shown 
in figure 1 . 

For central California Jepson (1911) says, '^ Forming patches in 
dry ground, common and widely distributed in barren valley fields." 
It is also a foothill species; in speaking of Nevada M. E. Jones, in an 
unpublished note, states that it occurs in juniper and oak zones. For 
the range from northern California to Washington, Howell tells us that 

1 The description of the plant and the account of its distribution were prepared by W. W. Eggleston, 
of the Bureau of Plant Industry. 



4 BULLETIN 969, U. S. DEPARTMENT OF AGRICULTURE. 

"it is found along streams." It follows the valley of the Columbia 
River and its tributaries into Idaho, where it is occasionally found at 
an altitude of 6,000 feet. It occurs on the Warner Mountains, Modoc 
County, Calif., at that altitude, but usually in other parts of the 
United States it is at much lower altitudes. 

Like A. galioides it quite plainly thrives best in newly disturbed 
soil, but there is no evidence that it is spreading as a weed so rapidly 
as A. galioides. In comparison with A. galioides it is a larger plant, 
with wider leaves and flower clusters in a terminal corymb. 




FiF. 1. — Distribtuion o! A-fclepiaf: mexkana in the United States. 
EXPERIMENTAL WORK. 

During the summer of 1920, 19 feeding experiments with Asclepias 
mexicana were made. In all cases sheep were used, the material 
being fed with the balling gun. In 5 cases the whole top — leaves, 
stems, and flowers — was fed; in 5 other cases leaves and in 9 cases 
stems were used. The material was obtained through the kindness 
of Prof. H. M. Hall, of the Carnegie Institution, and was collected 
at Mount Diablo, Solano, Hollister, and between Newman and Dos 
Palos, all in California. While dried plant was fed, the dosage is 
given in terms of green plant for 100 pounds weight of sheep, 75 per 
cent being allowed for loss in drying. 

In Table 1 the experiments with this plant are summarized. 



Bui. 969, U. S. Dept. of Agriculture. 



Plate I. 




ASCLEPIAS MEXICANA, SHOWING MANNER OF FLOWERING AND THE FRUIT. 



Bui. 969, U. S. Dept. of Agriculture. 



Plate II. 




Fig. I.— Sheep 610. July 7, 1.20 p. m. 
Weakness in Hind Legs, More 
Pronounced on One Side. 



Fig. 2.— Sheep 610. July 9, 4.05 p.m. 
Position After Falling. 




Fig. 3.— Sheep 610. July 9, 4.12 p. M. 
Standing with Difficulty. 



Fig. 4.— Sheep 610. July 10, 
9.50 A. M. A Characteristic 
Attitude when Attempting to 
Stand. 




Fig. 5. -Sheep 590. September I, 
8.50 a.m. The Animal in aSpasm 
in Opisthotonos Position. 



Fig. 6.-SHEEP 590. September I, 
9.10 a.m. TheAnimal in aSpasm. 



MEXICAN WHORLED MILKWEED AS A POISONOUS PLANT. 



T.'i.BLE 1. — Suvimary of feeding experivients with Asdepias mexicana (air-dried plant fed 
with balling gun; no remedies used). 



Animal. 


Date of 
feeding. 


Part of plant used. 


Weight 

of plant 
estimated 

as green 
plant 

per 100 
pounds of 

animal. 


Result. 




Designation . 


Weight. 


Place and date of 
plant collection. 


Sheep 582 

Sheep 588 

Sheep 595 

Sheep 603.... 
Sheep 607.... 

Sheep 610 

Sheep 563 ... . 

Sheep 579 

Sheep 5S1 

Sheep 584.... 
Sheep 586 

Sheep 595 

Sheep 568 

Sheep 575 

Sheep 580.... 

Sheep 590 

Sheep 595 

Sheep 602.... 
Sheep 603 


Pounds. 
98 

93 

78 

97.75 
93 

81.75 
102.5 
86 
75.5 

82 
128 

81 
86 
72.25 

69 

103. 5 

92 

93. 25 
106.5 


1920. 
June 21 

June 23 

June 28 
July 1 
July 6 

...do 

July 8 
July 13 
July 14 

July 16 
July 20 

July 22 
Aug. 10 
Aug. 13 

Aug. 23 

Aug. 31 
Sept. 3 
Sept. 9 
Sept. 13 


Leaves 


Pounds. 
0.176 

. 353 

.529 
.705 
.705 

.882 
.882 
1.058 
1.323 

1.411 

1.764 

1.481 
.882 
1.058 

1.235 

1.543 
1.323 

2.205 
3.527 


Not sick 


Mount Diablo,Calif., 


.do 


fall of 1919. 
Solano, Calif., fall of 


.do 


do 


1919. 
Do. 


do 


do 


Do. 


Stems. 


Not sick 

Death 


Do. 


Leaves 


Do. 




Not sick 

. ...do 


Do. 


.do 


Do. 


.do 


Slight sickness. 

Not sick 

Slight sickness. 

Not sick 


Mount Diablo,CaIif. 


. .do 


fall of 1919. 
Do. 


do 


Mount Diablo and 


.do 


Solano, Calif., fall 
of 1919. 
Solano, Calif., fall of 


Leaves, stems, and 

flowers. 
Leaves, stems, and 

flowers. Large 

proportion of 

stems. 
Leaves, stems, and 

flowers, 
.do 


1919. 


do 

do 

Death 


July, 1920. 
Do. 

Do. 
Do. 


.. .do 


do 

Symptoms 

Possible symp- 
toms. 


Do. 


Stems. 


Do. 


do 


Do. 



TYPICAL CASE, SHEEP 610. 



Sheep 610 was a ewe weighing 81.75 pounds at the time of the exper- 
iment. wShe had not been used in any other experimental work. To 
this animal was given by the balling gun, between 5.07 and 5.15 p. m., 
on July 6, 87.75 grams of dry Asdepiafi mexicana leaves, an equivalent 
of 0.882 pound of green plant per 100 pounds of animal. When 
observed at 6.25 a. m. July 7 she was unsteady, staggering as she 
walked about. At 7.20 a. m. the unsteadiness had increased in a 
marked degree. At 8.59 a. m. her temperature was 105° F., pulse 
152, respiration 36. When observed between 1.10 and 1.30 p. m. 
she could move about without falling, but staggered, and when mov- 
ing about went with a sidling gait, the weakness being most pro- 
nounced in the hind legs. Plate II, figure 1, shows very clearly this 
weakness in the hind legs. This condition continued during the after- 
noon and evening. At 5.33 p. m. the temperature was 103, pulse 
140, and respiration 44. 

On July 8 at 8.30 a. m. she appeared somewhat better than on the 
preceding day. At 9.36 a. m. the temperature was 102.4, pulse 120, 



6 BULLETIN 969, U. S. DEPARTMENT OF AGRICULTURE. 

respiration 40, the pulse being somewhat weak and irregular. Dur- 
ing the day her condition remained very much the same, and at night 
she seemed to be somewhat stronger than in the morning. 

On July 9 at 9.22 a. m. the temperature was 102.5, pulse 120, 
respiration 56. The pulse was rapid, weak, and regular. The animal 
still was weak and staggered, as in the preceding day. At 4.07 p. m. 
the sheep was watered, and while being brought in fell and was able 
to stand only with assistance. When placed in her pen she fell, 
coming down flat on the ventral surface, with her hind legs stretched 
out behind. (See PI. II, fig. 2.) She was able soon to get upon her 
feet, but stood with difficulty, as shown in Plate II, figure 3. At 
4.27 p. m. she was down again in the same position as shown in Plate 
II, figure 2. At 4.46 p. m. her temperature was 103, pulse 170, res- 
piration 60. Respiration was irregular ,and she was unable to stand 
on her feet very long. 

On July 10 at 9.03 a. m. the temperature was 102.1, pulse 140, 
respiration 80. During the day the animal continued much as on 
the preceding day, but apparently was somewhat stronger. At 9.45 
in the morning she was taken out of the pen and placed in the yard, 
where she could get at grass, but ate very little. Plate II, figure 4, 
shows the attitude assumed by the animal. At 4.21 p. m. the tempera- 
ture was 104.3, pulse 128, respiration 82. At 7.20 p. m. she suddenly 
fell in a spasm, drawing the head back in a position of opisthotonos and 
using the legs vigorously. She was unable to get upon her feet until 
7.24 p. m., when she stood with legs spread apart, head low and 
stretched forward. At 7.27 p. m. she stepped back with her head 
held high, fell over on the left side, groaned, struck her head upon 
the ground repeatedly, then threw the head back in a clonic spasm fol- 
lowed by a tonic spasm. At 7.33 p. m. she was lying with head partly 
raised and shaking as if with palsy. At 7.34 p. m. she rose to her feet, 
but at 7.35 fell with the forefeet spread out. At 7.38 p. m. she was upon 
her feet again, and was able to stand in a very uncertain way, but 
was down again at 9.35 p. m., the mouth moving as if chewing. She 
was somewhat salivated at this time. At 10 p. m. she was still down, 
but with the head erect, moving from side to side. At 10.15 p. m. she 
was going through a series of clonic spasms. At 11 p. m. she was 
dead, having struggled more or less violently just before death. 

No good pictures were taken of this animal during the period of 
spasms, but Plate II, figures'5 and 6, which were taken of Sheep 590, 
show some of the characteristic attitudes assumed by animals in the 
stage of spasms. Figure 5 shows very clearly, in the condition of 
the ground of the corral, how the animal when down goes through a 
series of motions. This animal differed somewhat from others in 
that at no time was there an extremely high temperature. 



MEXICAN WHOHLED MILKWEED AS A POISONOUS PLANT. 7 

RESULTS OF EXPERIMENTAL WORK AND CONCLUSIONS. 

SYMPTOMS. 

The symptoms exhibited by poisoned animals paralleled very 
closely those seen in cases poisoned by the other whorled milkweeds, 
A. galioides, A. pumila, and A. verticillata var. geyeri, differing little 
except in degree. 

There was depression with staggering, the weakness being espe- 
cially marked in the hind legs. As in the A. pumila cases, while 
standing the animals frequently held the head rather high with the 
nose extended forward. The pulse was always high and rather weak. 
These were the only symptoms in the milder cases, if we except a 
slightly higher temperature. 

In the severe cases the stage of weakness was followed by a stage 
in which there were spasms, mostly clonic, like those exhibited by 
animals poisoned by A. galioidefi. The animal would throw itself 
about violently, sometimes showing an opisthotonic position in its 
spasms, sometimes drawing the head toward the chest. Walking 
and running movements, while lying on the side, accompany the 
spasms, and frequently there are convulsive movements of the jaw. 
Two of the fatal cases were salivated and one was bloated. The 
animals groan, and in breathing there is frequently a marked interval 
between inspiration and expiration, the expiration being more or less 
violent. The temperature rises during the spasms. In Sheep 590 
it reached 107.7° F. and in Sheep 595, 10G.4° F. These maximum 
temperatures were taken just after the death of the animals. In 
this connection it is interesting to note that temperature observa- 
tions on A. galioides showed a decline in temperature before death 
(Bui. 800, p. 26), while in the A. mexicana and A. pumila cases (Bui. 
942, p. — ) the maximum was reached at the time of death. In all 
whorled-milkweed cases the prognosis of those which pass into the 
stage of spasms is bad. 

Only three of the A. mexicana cases showed spasms, and they died. 
Of the A. galioides cases described in Bulletin 800 only one. Sheep 
478, recovered after passing into the stage of spasms. Of the sheep 
poisoned by A. pumila only two exhibited spasms, and they died. 
Only three sheep were poisoned by A. verticillata var. geyeri; all re- 
covered, and two went into the stage of spasms. 

DELAY IN DEVELOPMENT OF SYMPTOMS. 

Table 2 shows the time which elapsed before the appearance of 
symptoms . 

The average time of 14 hours and 5 minutes between the time of 
feeding and beginning of symptoms is probably slightly high. A few 
sheep showed symptoms when first seen in the morning and may 
have been sick for some time previously. Then, too, in cases which 



BULLETIN 969, U. S. DEPARTMENT OF AGRICULTURE. 



^ 



come down somewhat slowly, as these do, it is not easy to distinguish 
the very beginning of symptoms. The shortest time was 12 hours 
and 45 minutes, occurring in three cases, and they were sick when 
first observed in the morning following the feeding. 

Table 2. — Time elapsed between feeding of plant and development of siimjitnwx. 



I 
I 



Animal. 



Quantity 
per 100 
pounds 

of 
animal. 



.(June 28). 
(Tulyl)... 



Sheep 588. 
Sheep 595 ( 
Sheep 603 i 
Sheep 610. 

Sheep 581 

Sheep 586 

Sheep 568 

Sheep 575 

Sheep 580 

Sheep .590 

Sheep 595 (Sept. 3).. 

Sheep 602 

Sheep 603 (Sept. 13). 



Pounds. 
0.353 
0.529 
0. 705 
0.882 
1.323 
1.764 
0.882 
1.058 
1.235 
1.543 
1.323 
2.205 
3. 527 



Part of plant used. 



Leaves 

do 

do 

do 

Stems 

....do 

Leaves, stems, and flowers. . 

do 

do 

do 

do 

Stems 

do 



Result. 



Sickness 

do 

do 

Death 

Slight sickness. 

do 

Sickness 

....do 

...-do 

Death 

do 

Symptoms 

do 



Time 

elapsins 
before 
symp- 
toms. 



//. m. 

\i 40 

14 45 
16 25 

13 10 

15 10 
15 37 

14 51 
12 45 
12 45 
12 55 
12 45 

12 .50 

13 49 



14 



The time elapsing before symptoms in A. galioides was 14 hours 
and 6 minutes; in A. pumila 16 hours and 17 minutes; and in .4. 
verticillataY&r. geyeri 16 hours and 47 minutes. 

The average time in all the species of whorled milkweed was much 
the same. There was, however, much more variation in limits of 
time in the other species. A. galioides YSiTied from 2^ to 21 hours, 
A. pumila from 12 hours and 37 minutes to 30 hours and 15 minutes, 
and A. verticillata var. geyeri from 14 hours and 10 minutes to 20 
hours and 35 minutes, while xi. mexicana varied only from 12 hours 
and 45 minutes to 16 hours and 25 minutes. 

A comparison of these figures with those in Fleming and co-authors 
(1920, pages 15 to 17), shows a good agreement in averages. In Table 
I on feeding of green plant the average time from feeding to symptoms 
was 15 hours and 27 minutes. The combined average of Tables II 
and III of feedings of dry plant was 14 hours and 48 minutes. This 
latter average is obtained on the assumption that the time of feeding 
of Sheep No. 8 was probably 2.50 p. m. instead of 2.50 a. m., as given 
in the Nevada bulletin. 

There is, however, a wider range of variation than in the work at 
the Salina Experiment Station, the minimum being 1 hour and 50 
minutes, and the maximum 23 hours and 20 minutes. The large max- 
imum figures are doubtless explained, in part at least, as stated in the 
Nevada bulletin, by the fact that no night observations were made, 
so that in many cases the time of the beginning of symptoms was not 
noted. The minimum figures in most cases are correlated with very 
heavy doses. 



MEXICAI^^ WHOELED MILKWEED AS A POISONOUS PLANT. 



DURATION OF SICKNESS. 

Table 3 shows the length of time the sickness continued in cases 
which recovered. The times were computed from the first symtom 
noted to the last. The actual periods of illness doubtless were some- 
what greater, for as the animals were not generally under constant 
observation neither the beginning nor the ending of sickness would 
be recorded with much accuracy. The figures, however, will give a 
good general idea of the duration of illness and can be compared 
with the results in the other whorled milkweeds, as they were obtained 
in the same manner. 

Table 3. — Duration of sickness in cases of recorery. 



Animal. 



Sheep 588 
Sheep 595 
Sheep 603 
Sheep 581 
Sheep 5'^6 



Duration 

of symp- 

toins. 



H. m. 

50 10 

48 20 

48 1 

7 30 

7 54 



Animal. 



Duration 
' of symp- 
I toms. 



Sheep 568 
Sheep 575 
Sheep 580 
Sheep 602 
Sheep 603 



H. m. 
95 54 

73 45 

74 
5 40 

(1) 



' One observation only. 

Of the three animals that died, Sheep 610 was sick 89 hours and 
35 minutes; Sheep 596, 14 hours and 56 minutes; and Sheep 595, 
15 hours and 15 minutes. 

The shortest period of illness was in Sheep 602 — 5 hours and 40 
minutes — and the longest in Sheep 568 — 95 hours and 54 minutes. 
As in other whorled-milkweed cases the symptoms continued longest 
in the more pronounced cases. 

The average period of sickness was 45 hours and 42 minutes. 

It was shown in Bulletin 942, United States Department of Agri- 
culture, pages 12-13, that the average period of sickness in sheep 
poisoned by A. galioides was 4 hours and 39 minutes; by A. pumila 
it was 48 hours; and by A. verticillata var. geyeri it was 56 hours. 

It is evident that the period of sickness in the A. mexicana cases 
corresponded fairly well to that found for A. pumila and A. verti- 
cillata var. geyeri, and like them differed widely from the A. galioides 
cases. 

In the experiments of Fleming and co-authors (1920) the average 
duration of sickness in the sheep that recovered, as given in Tables I, 
II, and III, was 39 hours and 37 minutes, while in the cattle as 
given in Tables VII and IX it was 34 hours and 1 minute. 

AUTOPSY FINDINGS. 

Autopsies were made upon the three animals that died. Two of 
the three were bloated. The lungs were congested in all. In 
two the thymus was congested, and in the third there were petechiae 
in this organ. Petechiae were present on the hearts of two. 



10 BULLETIN 969, U. S. DEPARTMENT OF AGRICULTUEE. 

In one animal congestion was present in the duodenum, jejunum, 
and ileum; in another this condition was found only in the ileum 
and cecum; while the third had a hemorrhagic area in the duodenum 
and minute hemorrhages in the ileum. The pancreas was congested 
in one. The livers of two were noted as pale. The kidneys were 
congested in one and hyperemic in another. 

In general, the autopsy findings corresponded very well to 
those seen in animals poisoned by the other whorled milkweeds. 

PATHOLOGICAL CHANGES IN TISSUES. 

The autopsies in 2 of the 3 sheep killed with Asclepias mexicana 
in 1920 were commenced within 30 minutes after their deaths. 
These were Sheep 590 and 595. Sheep 610 was dead much longer be- 
fore the autopsy and well-marked post-mortem changes had occurred 
in the tissues. The following description of the pathological changes 
is based on Nos. 590 and 595. The material was fixed in 10 per cent 
formalin and stained with Delafield's hematoxylin and eosin, except 
specimens of the cerebellum of Sheep 590, which were fixed in alcohol 
and stained with various nerve-cell stains. Formalin-fixed material 
of the cerebellum of both sheep was stained with toluidin blue, Nissl 
stain, polychrome methylene blue, neutral red, and other stains. 

Liver. — The hepatic cells in both animals had undergone well- 
marked degenerative changes which differed slightly in the two 
cases — those of Sheep 590 were very fatty, the nuclei, however, 
taking the stain well, while the hepatic cells in the liver of Sheep 
595 had a very granular and swollen cytoplasm. In the liver of 
Sheep 595, too, certain hepatic cells, singly and in groups, showed 
marked affinity for eosin. In the capillaries of the liver lobules of 
both there was an unusually large number of leucocytes, especially 
polymorphonuclear. These were often in small clumps, covering 
the area of half a dozen or more hepatic cells in the liver section 
of Sheep 590. 

The livers of both animals had congested and edematous areas 
with some small hemorrhages. This was most noticeable in one 
preparation of the liver of wSheep 590, where the congestion was 
severe just beneath the capsule and had compressed the liver cords. 

Many veins, both portal and hepatic, were empty, while others 
contained much granular detritus, brownish pigment granules, de- 
generate erythrocytes, and sometimes endothelial cell nuclei and 
numerous leucocytes. Rarely hepatic cells were found in the sub- 
lobular veins. The wall of the congested portion of the cormnon 
bile duct in both cases had undergone necrotic changes extending 
well into the mucosa. The epithelial layer was gone. 

Kidneys. — In both kidneys there was congestion of the straight 
veins of the medulla and areas of capillary congestion in the cortex 
with a few small hemorrhages betweeen the tubules. The principal 



MEXICAN WHORLED MILKWEED AS A POISONOUS PLANT. 11 

change had occurred in the epithehal cells of the convoluted tubules 
and consisted of cloudy swelling. Other cells were present, singly 
and in groups, the cytoplasm of which had an unusual affinity for 
eosin. Such cells contained shrunken nuclei which stained darker 
and more uniformly than normal. Other nuclei were very faint or 
had entirely disappeared. The lumina in may places contained a 
considerable quantity of disintegrated cytoplasm and sometimes 
free epithelial cell nuclei. In congested areas frequently epithelial 
cells of the tubules contained considerable brownish granular pig- 
ment. Granular material, pigment, and an abnormal number of 
leucocytes were found in some veins. Some distended capillaries 
in the sections from the kidneys of Sheep 595 were occupied by 
hyalin areas which stained strongly with eosin. 

Other tubules than the convoluted, as the ascending limb of Henle, 
showed similar though less pronounced changes. 

Heart. — The myocardium of the left ventricle showed changes like 
those occurring in some of the A. galioides cases, but were not so 
marked. In many areas the capillaries were distended with blood 
and very prominent between the muscle fibers. The tissue was 
mildly edematous. In most areas the muscle fibers had a less dis- 
tinct cross-striated appearance and a more granular cytoplasm than 
normal. A few minute hemorrhages appeared. Very few leuco- 
cytes were present. The red blood corpuscles stained well and ap- 
peared to be normal. 

Lungs. — There was a mild inflammation of the lungs. The capil- 
laries were engorged and distended, in many places nearly filling 
the alveoli. Numerous leucocytes, mostly polymorphonuclear, 
were scattered throughout the tissue. Many epithelial cells of the 
alveoli appeared swollen or vacuolated and their nuclei were irregu- 
lar in outline. The epithelial cells of the bronchioles were swollen or 
in the process of disintegration, the bronchioles containing much 
granular material, many nuclei of exfoliated cells, and sometimes 
erythrocytes. 

In some veins there were areas composed of normal and pale 
erythrocytes, much finely granular material, and a considerable num- 
ber of leucocytes, mostly polymorphonuclear. A considerable cjuantity 
of brownish pigment was distributed throughout the sections. 

Thyroid. — Little that was abnormal was found in the thjrroid 
glands. This organ was not congested in either animal, though 
some brownish pigment and pale erythrocytes were found. In a 
few capillaries the erythrocytes appeared to have fused, forming 
hyalin thrombi. In the A. galioides cases this gland was almost 
always severely congested. 

Thymus. — Congestion had occurred in the thymus in each of the 
two sheep, though this was most pronounced in the thoracic portion 
of the gland of Sheep 595, where extensive hemorrhage into the 



IS BULLETIN 96^, U. S. DEPARTMENT OF AGRICULTURE. ^ 

glandular tissue had occurred, especially into the medullary portion 
of the lobules. Around some blood vessels in the thymus of vSheep 
595 there were many large, coarsely granular cells, which had a 
strong affinity for eosin. These closely resembled eosinophiles, 
except that many were rather large and had nuclei almost circular 
in outline. The lymphoid cells appeared to be normal. vSome 
brownish pigment was present, and in a few small veins in Sheep 
590 the erythrocytes appeared as though fused. 

Spleen. — In the specimen from the spleen of Sheep 590 there were 
small areas of congestion in the splenic pulp. The samples from 
Sheep 595 contained very little blood. In both cases some brownish 
pigment was present, and in the tissue from Sheep 590 some cavernous 
veins contained pale and disintegrated erythrocytes. 

Lymph glands. — A mesenteric lymph gland and a small group of 
hemolymph nodes from Sheep 590 were studied. There was no 
indication of an effect on l3niiphoid cells, but some erythrocyte de- 
struction was apparent. In the lymph gland many endothelial cells 
contained ingested erythroc3^tes in various stages of destruction. 
In the hemolymph nodes there were areas where practically all 
erythrocytes stained very poorly. 

Alimentary tract. — Specimens from the abomasmn, duodenum, 
jejunum, ileum, and cecum were studied. There was a well-marked 
infiltration of leucocytes into the mucosa of the various portions 
of the small intestine and the one sample of cecum examined. These 
were of the polymorphonuclear and plasma cell types. Congestion 
accompanied with diapedesis of erythrocytes and edema was present 
in some places, and in all portions of the small intestine the villi and 
that portion of the mucosa next the lumen of the intestine were more 
necrotic than one would expect in an animal that had been dead so 
short a time. 

The glandular epithelial cells were swollen and many small nuclei, 
probably of leucocytes, were very commonly lying in the epithelial 
layer and sometimes in the lumen of the gland. Many such nuclei 
were degenerated. In places the veins contained pale erythrocytes, 
granular material, and sometimes a fibrinlike network and numerous 
leucocytes. 

The lymp^x nodes of the ileum of Sheep 590 were edematous and 
contained an unusual number of phagocytic cells, which had in- 
gested erythrocytes and pigment or partially digested granules of 
nuclear material. 

Nervous tissue. — Specimens of cerebrum, cerebellum, medulla, 
and cervical and lumbar portions of the spinal cord were examined. 
None of these was severely congested, though minute hemorrhages 
were formed in various places. These were presumably due to diape- 
desis of the red corpuscles, as ruptured vessels were not found. In 
most hemorrhages the erythrocytes were confined to the perivascular 



MEXICAIs^ WHOBLED MILKWEED AS A POISONOUS PLANT. 



13 



lymph space, but in some instances they had penetrated into the 
surrounding tissue. Pericellular lymph spaces, in many if not most 
instances, were enlarged. The nerve cells had undergone fatigue 
changes similar to those found in the A. pumila and A. galioides 
cases. This consisted m a breaking up of the Nissl bodies or a 
marked reduction in the chromatin content of the cell. 

In general the pathological conditions of the A. mexicana 
cases are the same as those found in A. galioides and A. pumila 
poisoning, and consist mainly of capillary congestion and degenerative 
changes in certain tissues. Areas of congestion were found in the 
liver, kidneys, heart, muscle, lungs, thymus, and mucosa of portions 
of the intestines. The cells of the liver, kidney tubules, heart muscle, 
intestinal glands, and certain nerve cells showed degenerative changes 
which may probably be accounted for by excessive functional 
activity. Fatigue effects were most apparent in the heart-muscle 
fibers and nerve cells. The presence of an abnormal number of 
leucocytes in the liver, intestinal mucosa, and lungs is probably due 
to the degenerative changes of the tissue cells. 

Degeneration of hepatic cells, epithelium of kidney tubules, and 
intestinal mucosa appears to be more pronounced in the A. mexicana 
cases than in those animals killed by A. galioides. On the other 
hand the degeneration and capillary congestion of heart muscle, con- 
gestion of the nervous system, and fatigue effects on the Purkinje 
cells of the cerebellum were much more pronounced in the A. galioides 
cases. 

The changes found in the tissues of the A. pumila cases correspond 
very closely to those described for the A. mexicana cases. The effects 
of these two species of plants would appear to be practically the 
same, but those produced by the more toxic A. galioides differ slightly 
in degree. 

TOXIC AND LETHAL DOSES. 

The balling gun was used in all the experimental work from which 
the dosage was computed, so that the animals probably received the 
material in a somewhat shorter time than they would when feeding 
on the range. It would be expected on this account that under range 
conditions the dosage, in the majority of cases, would be larger. 

Wliole plant. — Table 4 shows the dosage when the animals received 
the whole plant. 

Table 4. — Dosage of A. mexicana, whole plant. 



Animal. 



Sheep 568 
Sheep 575 
Sheep 580 



Quantity 

fed per 100 

pounds of 

animal. 



Pounds. 
0.882 
1.058 
1.235 



Result. 



Sickness, 
do. 
do. 



Animal. 



Sheep 590. 
Sheep 595. 



Quantity 
fed per 100 
poundsof 
animal. 



Pounds. 
1.543 
1.323 



Death. 
Do. 



14 



BULLETIN 969, U. S. DEPARTMENT OF AGRICULTURE. 



The minimum lethal dose for these animals was 1.323 pounds. 
This is probably a rather accurate figure, for Sheep 575 received 
1.058 pounds and Sheep 580 1.235 pounds without fatal results in 
either case. The minimum toxic dose was 0.882 pound. No experi- 
ments were made with slightly smaller doses, but as Sheep 568 was a 
mild case this dose must be not far from the minimum of toxicity. 

Leaves. — Table 5 shows the dosage when leaves only were fed. 

Table 5. — Dosmjc of A. vwxicana, leaves only. 



Animal. 



Sheep 582 
Sheep 588 
Sheep 595 



Quantity 

fed per 100 

poimdsof 

animal. 



Pounds. 
0.176 
0.353 
0.529 



Result. 



Animal. 



Not .sick. 
Sickness, 
do. 



Sheep 603. 
Sheep 610. 



Quantity 
fed per 100 
pounds of 
animal. 



Pounds. 
0.705 
0.882 



Result. 



Sickness. 
Death. 



As Sheep 603 was not very sick on 0.705 pound, it is probable that 
0.882 pound, which caused the death of Sheep 610, is close to the 
lethal dose. While 0.353 pound was the smallest dose producing 
sickness, it is possible that it is not the minimum, as there were no 
tests between that and 0.176 pound which was given \vithout effect 
to Sheep 582. 

Stems. — Nine sheep were fed on stems from which all leaves had 
been removed, as shown in Table 6. 

Table 6. — Dosage of A. mexicana, stems only. 





Quantity 




Quantity 




Animal. 


fed per 100 
pound.sot 
animal. 


Result. 


Ammal. 


fed per 100 
pounds of 
animal. 


Result. 




Pounds. 






Pounds. 




Sheep 607 


0.705 


Not sick. 


Sheep 586 


1.764 


Shght sickness. 


Sheep 563 


0.882 


do. 


Sheep 595 


1.481 


Not sick. 


Sheep 579 


1.0.58 


do. 


Sheep 602 


2.205 


Symptoms. 


Sheep 581 


1.323 


Slight sickness. 


Sheep 603 


3.527 


Possible symptoms. 


Sheep 584 


1.411 Not Sick. 









The smallest quantity of stems fed which produced symptoms was 
1.323 pounds. As compared with this, Sheep 584 and Sheep 595 
•showed no effect from 1.411 pounds and 1.481 pounds, respectively. 
The table shows also that as much as 3.527 pounds was fed to Sheep 
603 with only slight effect. No animals were killed by the feeding of 
stems. It appears then that as little as 1.323 pounds may produce 
slight symptoms. The lethal dose has not been determined, but it 
probably is several times this quantity. 

For comparative purposes the toxic and lethal doses of whole plant, 
leaves, and stems to 100 pounds of animal may be tabulated as 
follows : 



Minimum toxic 

dose. 

Pounds. 

Loaves 0. 353 

Whole lop 0. 882 

Stems 1. 323 



Minimum lethal 

dose. 

Pounds. 

0.882 

1.3 

Moro than 3. 5 



MEXICAN WHORLEU MILKWEED AS A POISONOUS PLANT. 



15 



It is evident that the toxic principle of the plant is found very 
largely in the leaves. 

A comparison of the preceding figures with the toxic and lethal 
dosages for sheep given in Nevada Bulletin 99 can be made by 
reducing the dosage in Table I, page 15, of the Nevada bulletin to 
pounds per hundredweight of animal. It will be seen that the 
smallest toxic dose, 0.537 pound per hundredweight in the case of 
Sheep 25, is somewhat smaller than the figures obtained in our 
experiments. It may be noted, too, that Sheep 75 received 0.555 
pound without effect and that the smallest lethal dose was 2.3 
pounds in the case of Sheep 6. 

If the data of Table II of the Nevada bulletin giving the feedings 
of air-dried plant are similarly reduced to a dosage for 100 pounds 
of animal and an allowance of a loss of 75 per cent by evaporation is 
made, as in our work, the smallest lethal dose would be 2.059 pounds 
in Sheep 72 and the smallest toxic dose 1 pound in Sheep 59. 

The dosage in Table III (Nevada) of material which had dried in the 
field is somewhat larger, the lethal dose, 7.5 pounds in Sheep 43, 
being especially large. This is probably accounted for by the fact 
that there had been some loss of leaves in the field, for it is showTi on 
page 14 of this paper that the leaves are especially toxic. 

Table 7 shows the comparative toxicity of the different whorled 
milkweeds : 

COMPARATIVE TOXICITY OF SPECIES OF WHORLED MILKWEED. 

Table 7. — Comparative toxicity of different species of whorled milkweed. 



Species. 



Miiiimiun 

toxic dose 

of whole 

plant. 



Minimum 

lethal dose 

of whole 

plant. 



Minimum 
toxic dose 
of leaves. 



Minimum 
lethal dose 
of leaves. 



Mmimum 
toxic dose 
of stems. 



A. mexicana 

A. galioides 

A. piunila 

A. verticillata var. geyeri 



Pounds. 

0.S82 
0.22 
0.787 
2.205 



Pounds. 
1. .323 
0.22 
2.165 



Pounds. 
0.353 
0. 138 



Pounds. 
0.882 
0.138 



Pounds. 
1.323 
1.543 



While too much reliance must not be placed on this table as show- 
ing exact relationships, nevertheless it indicates, in a general way, 
the relations between the species. So far as the whole plant is con- 
cerned, A. mexicana and A. pumila are about equally toxic. 
A. galioides is 4 times as toxic as these two species and 10 times as 
toxic as A. verticillata var. geyeri. As regards the lethal dose of the 
whole plant, A. galioides is 6 times as toxic as A. mexicana and 9.84 
times as toxic as A. pumila. 

With relation to the leaves, A. galioides and A. verticillata var. 
geyeri compare much as in the whole plant; that is, in a ratio of about 
1 to 10. The ratio of A. galioides to A. mexicana, however, is 1 to 
2.56 instead of 1 to 4, as in the whole plant, while the lethal dose is in 



16 BULLETIN 969, U. S. DEPARTMENT OF AG". 



022 265 963 



the ratio of 1 to 6. There is an apparent anomaly in the dosage of 
the stems in that the much more poisonous A. galioides requires a 
slightly larger dose than A. mexicana. This may be explained in part 
by the fact that the stems of both plants are only slightly toxic, but 
there is also some reason for thinking that the chemical study of 
these plants, the results of which will be published later, will offer an 
adequate explanation. 

ANIMALS AFFECTED. 

The experiments reported in this bulletin were on sheep only. 

Fleming and his co-authors (1920) have shown that cattle are poisoned 

with a somewhat similar dosage. From the fact that A. mexicana 

produces effects strictly comparable with those caused by A. galioides 

and that it has been shown in a previous paper (U. S. Department of 

Agriculture Bulletin 800) that A. galioides poisons horses as well as 

sheep and cattle, it is fair to conclude that A. mexicana will have the 

same effect. 

SUMMARY. 

Asclepias mexicana (the Mexican whorled milkweed), a plant 
growing on the Pacific slope, and especially in California and Nevada, 
has long been considered a stock-poisoning plant, particularly dan- 
gerous to sheep; but there has not been definite knowledge of the 
symptoms produced by it or of its toxic dosage. 

It produces effects very similar to those produced by the other 
whorled milkweeds, A. galioides, A. pumila, and A. verticillata var. 
geyeri. 

In toxicity A. mexicana is about equal to A. pumila and about one- 
foiu-th as toxic as A. galioides. The lethal dose is six times that of 
A. galioides, but about one-half that of ^4. pumila. 

LITERATURE CITED. 

Chesnut, V. K. 1898. Preliminary catalogue of plants poisonous to stock. Annual 

report of the Bureau of Animal Industry. Pp. 387-420. 
Fleming, C. E. 1920. Project 22— Poisonous range plants. Rpt. Nev. Exp. Sta., 

p. 41. 
Fleming, C. E., Peterson, N. F., Miller, M. R., Vawter, L. R., Wright, L. H. 

1920. The narrow-leaved milkweed and the broad-leaved or showy milkweed. 

Bui. 99, NeA-ada Agricultural Experiment Station. 
Hall, H. M., and Hall, C. C. 1912. A Yosemite flora. Pp. 189-190. 
Hall, H. M., and Yates, H. S. 1915. Stcck-poisoning plants of California. Bui. 

249, Agricultural Experiment Station, Berkeley, Calif. 
Howell, T. J. 1901. A flora of Northwest America. P. 441. 
Jepson, Willis Linn. 1911. The flora of Western Middle California. P. 323. 
Marsh, C. Dwight, Clawson, A. B., Couch, J. F., and Eggleston, W. W. 1920. 

The whorled milkweed (Asclepias galioides) as a poisonous plant. U. S. Department 
of Agriculture, Bulletin 800. 
Pammel, L. H. 19.10. A manual of poisonous plants. Parti, p. 130. 

WASHINOTOX : GOVERNMENT PRINTING OFFICE : 1921 



